In many types of electronic applications, especially those that contain power amplifiers, signal distortion plays a significant role. Power amplifiers are used in communication systems, for example, to increase the signal strength of wireless transmissions between a base station and a mobile device such as a cellular telephone or PDA or between base stations. A radio frequency (RF) power amplifier can be disposed in either or both the transmitter or receiver path. Such power amplifiers are used in RF transmitters in which digitally modulated carriers such as TETRA, iDEN, GSM, and CDMA are employed.
In an ideal system, the RF power amplifier is linear and thus the ratio of the output power to the input power does not vary with the input power. However, the RF power amplifier and accompanying circuitry in the signal path are non-ideal and subject to nonlinearities such as intermodulation products and DC offsets that add noise and cause distortion. This is problematic as restrictions on out-of-band emissions may be severe (to the order of −60 dBc to −70 dBc relative to the power in adjacent frequency channels). Hence, different techniques have been developed for improving linearization of the RF power amplifiers. The most prevalent technique is enclosing the power amplifier within a Cartesian feedback loop.
As above, although both base stations and communication devices contain one or more RF power amplifiers and an associated linearization system, increasing the signal fidelity for base stations may be more problematic due to the higher amplification (and more dynamic range of amplification) used. Due to the wide range of technology serviced by base stations, it is desirous for base stations to simultaneously transmit multiple carriers. As the intermodulation between carriers in a single RF power amplifier is relatively large, this is usually accomplished using one RF power amplifier per carrier, resulting in multiple RF power amplifiers each serving a predetermined carrier. It is often attractive to use a single power amplifier for multiple carriers from a cost perspective. Presently-used Cartesian feedback systems provide insufficient linearization when a single multicarrier RF power amplifier is used, for example, in Terrestrial Trunked Radio (TETRA) system whose standard contains relaxed carrier bandwidths and tough restrictions on unwanted signal components.
It would thus be desirable to provide a system in which adequate linearization is provided when a single power amplifier is used over a wide range of carriers.